Semiconductor light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs) are among the most efficient and robust light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials, and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials.
A common problem with conventional semiconductor light-emitting devices is that the efficiency with which light may be extracted from such a device is reduced by total internal reflection at interfaces between the device and the surrounding environment followed by reabsorption of the reflected light in the device. Such total internal reflection occurs because the index of refraction of the semiconductor materials from which the device is formed at the emission wavelengths of the device (n˜3.5 for III-Phosphide materials, for example) is larger than the index of refraction of the material, typically an epoxy or silicone (n˜1.5 or less), in which the device is packaged or encapsulated. Losses due to total internal reflection increase rapidly with the ratio of the refractive index inside the device to that outside the device.
Another aspect of conventional semiconductor light-emitting devices which may be disadvantageous for some applications is that the emission spectrum of such a device typically exhibits a single rather narrow peak (full width at half maximum of about 15 to about 50 nanometers, for example) at a wavelength (peak wavelength) determined by the structure of the light-emitting semiconductor device and by the composition of the materials from which it is constructed. Some applications require a broader emission spectrum than can be directly produced by a conventional light-emitting semiconductor device. For example, some lighting applications require the production of apparently white light. Moreover, the efficiency with which a conventional light-emitting semiconductor device generates light typically varies as the structure and composition of the device is changed to tune the narrow emission spectrum. Consequently, conventional light-emitting semiconductor devices may be unsatisfactory for applications requiring efficient generation of light at particular wavelengths.
One conventional approach to broadening or shifting the emission spectrum of light-emitting semiconductor devices involves using the emission of such a device to excite a phosphor. As used herein, “phosphor” refers to any luminescent material which absorbs light of one wavelength and emits light of a different wavelength. For example, blue light from a light-emitting semiconductor device may be used to excite a yellow emitting phosphor. The resulting yellow light may be mixed with unabsorbed blue light to produce white light. Light-emitting devices in which emission from a semiconductor light-emitting device is converted by a phosphor to another wavelength are typically termed “phosphor converted light-emitting devices.” Unfortunately, such phosphor converted light-emitting devices are typically not as efficient as is desired.
What is needed is a semiconductor light-emitting device having improved light extraction, improved phosphor conversion, or both.